Computational insights into the Multi-Diels-Alder reactions of neutral C60 and its Li+ encapsulated analogue: A DFT study

Diels-Alder cycloaddition reaction is helpful to produce covalent derivatives of fullerene with desirable electronic and physical properties. In the present venture, we have computationally investigated the reactivity of neutral C and its Li encapsulated derivative towards Multi-Diels-Alder (MDA) reaction with 1,3-butadiene, employing density functional theory (DFT). The computational reports available to date illustrate the functionalization of fullerene surfaces of neutral and encapsulated C (Ca and Sm) with two butadiene molecules. In this article, we aim to investigate whether more than two butadiene molecules can be attached to the fullerene surface or not. To do so, we have shown that the MDA reaction initiates with the formation of an encounter complex between the mono-functionalized fullerene product and the second butadiene molecule. In this context, two different approaches, namely ‘Direct’ and ‘Alternative’ have been considered based on the attachment of the second butadiene, i.e., whether it is attached to the opposite or adjacent position of the first functionalization, which eventually produces the same final product. We have explored the MDA reactions by considering a total of four diene molecules that can be embedded successfully on the fullerene surface, with each reaction step having a high degree of exothermicity, thus making the overall reaction thermodynamically facile. In harmony with the mono- and bis-cycloaddition reactions, for MDA reaction also, the positive impact of Li encapsulation for enhancing the reactivity of fullerene surface towards butadiene attachment is evident from our study. On-the-fly calculations also suggest the bond preference for [6, 6] connectivity than its [6, 5] counterpart, to be the suitable dienophile, just like the mono- and bis-functionalization reported earlier. Overall, the present study will foresee an extensive idea about the detailed mechanism of the MDA reaction on neutral C and Li@C that could encourage the scientists to perform the aforementioned reaction for other fullerene derivatives in the long run.